Methods and apparatus for a semiconductor device

ABSTRACT

A method for increasing carrier concentration in a semiconductor includes providing a group III nitride semiconductor device, determining a wavelength that increases carrier concentration in the semiconductor device, and directing at least one infrared light source, at the determined wavelength, into a semiconductor device excitation band.

BACKGROUND OF INVENTION

This invention relates generally to semiconductor devices and morespecifically to enhancing carrier concentration in a semiconductordevice.

Some known semiconductor devices, such as light emitting diodes (LED),are fabricated from a group III nitride material, such as, acompositional alloy of the nitrides of Indium, Aluminum, Galium(InAlGaN). Such devices are sometimes doped with an acceptor speciessuch as, but not limited to, magnesium (Mg) to increase free carrierconcentration and therefore increase electrical conductivity.

The group III-nitride system, GaN, AlN, InN and their solid solutions,is of increasing importance in the development of optoelectronic andelectronic semiconductor devices, such as light emitting diodes, lasersand heterojunction bipolar transistors. In some known semiconductordevices a p-type dopant, such as Mg, is used as a dopant for the groupIII-nitride materials. Mg exhibits a relatively large thermal activationenergy of approximately 200 milli-electron volts (meV). Having thisactivation energy may result in the ionization of only a few percent ofthe acceptor atoms in the material at room temperature as dictated bythe Fermi-Dirac statistical energy distribution. Thus, largeconcentrations of Mg may be required to achieve the p-type conductivitynecessary for many device applications.

In some known InAlGaN devices, using Mg as a p-dopant, the InAlGaNp-type materials may require thermal activation to dissociate Mg—Hcomplexes which form during the process of growing the p-AlInGaNmaterial. Even after disassociation removes the hydrogen, leaving Mgdopant atoms behind, the Mg acceptor levels are typically a few hundredmilli-electron volts (meV) from a valence band edge in the semiconductordevice. This problem may worsen in LED's and laser diodes, as theconcentration of aluminum is increased to move the emission wavelengthfrom blue-green wavelength into an ultraviolet wavelength. Therefore, atambient temperatures there are relatively few thermally activated holesavailable in the p-type AlInGaN material.

SUMMARY OF INVENTION

In one embodiment, a method for determining a wavelength to improvecarrier concentration in a semiconductor device is provided. The methodincludes providing a group III nitride semiconductor device andirradiating the semiconductor device with a broad band light source togenerate a semiconductor device response, wherein the semiconductordevice response includes at least one of a photocurrent in a p-typelayer and an electroluminescence emission from a light emitting device.The method also includes identifying at least one wavelengthcorresponding to an acceptor ionization in the semiconductor devicebased on at least one of the photocurrent and the electroluminescenceemission.

In another embodiment, a method for increasing carrier concentration ina semiconductor device is provided. The method includes providing agroup III nitride semiconductor device, determining a wavelength thatincreases carrier concentration in the semiconductor device, anddirecting at least one infrared light source, at the determinedwavelength, into a semiconductor device excitation band.

In a further embodiment, an apparatus for increasing carrierconcentration in a semiconductor device is provided. The apparatusincludes a group III nitride semiconductor device and at least oneinfrared radiation source optically coupled to the semiconductor device.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a method for determining a wavelength to increase carrierconcentration in a semiconductor device.

FIG. 2 is a method for increasing carrier concentration in asemiconductor device.

FIG. 3 is a pictorial view of an infrared light source and asemiconductor device in an exemplary embodiment of the invention.

DETAILED DESCRIPTION

FIG. 1 is a method 10 for determining a wavelength to increase carrierconcentration in a semiconductor device. Method 10 includes providing 12a group III nitride semiconductor device and irradiating 14 thesemiconductor device with a broad band light source to generate asemiconductor device response, wherein the semiconductor device responseincludes at least one of a photocurrent in a p-type layer and anelectroluminescence emission from a light emitting device. In oneembodiment, the broad band light source is an infrared light source,such as, but not limited to, a tunable infrared light source such as adispersive infrared spectrometer. In another embodiment, the broad bandlight source is an interferometer including a spatially oscillatingmirror, such as a Fourier transform infrared spectrometer (FTIR). In oneembodiment, the broad band light source is used to photoionize acceptordopant atoms, thereby forming a non-equilibrium excess of holes in thevalence band of the p-type material. Method 10 also includes identifying16 at least one wavelength corresponding to an acceptor ionization inthe semiconductor device based on at least one of the photocurrent andthe electroluminescence emission. In one embodiment, identifying 16 atleast one wavelength to enhance carrier concentration includes observingan enhanced response of some aspect of the semiconductor device, i.e.the enhancement caused by the increased hole concentration in the p-typematerial.

In one embodiment, providing 12 a group III nitride semiconductor deviceincludes providing a semiconductor device fabricated from at least onegroup III material including Boron, Aluminum, Gallium, Indium, andThallium, such as, but not limited to, an Indium Aluminum GalliumNitride (InAlGaN) device. In one embodiment, providing 12 a group IIInitride semiconductor device includes providing a semiconductor devicewith a bandgap width greater than 1.5 eV.

In one embodiment, the p-type semiconductor material in the device isdoped with a p-type dopant, such as, but not limited to Magnesium (Mg).In an alternative embodiment, the p-type semiconductor material in thedevice is doped with a material such that an acceptor and donor energylevel is greater than 40 meV. Irradiating 14 the semiconductor devicewith a broadband tunable dispersive IR or FTIR interferometric lightsource to generate a semiconductor device emission includes, but is notlimited to, radiating the semiconductor device with a Fourier transforminfrared (FTIR) spectroscopy device. The FTIR device measures thedominant energies of crystal lattice vibrations, i.e. phonons, crystalintersubband transitions, local impurity vibrational modes, and anacceptor ionization of the semiconducting samples. FTIR spectrometersnormally record the interaction of infrared radiation with thesemiconductor device to measure the frequencies at which the sampleabsorbs the radiation and also the intensities of the absorptions.

In an exemplary embodiment, the magnitude of photocurrent in a layer ofp-type material is measured as a function of infrared wavelength,wherein the preferred wavelength is the wavelength of peak photocurrent,and the resulting infrared spectrum is the acceptor ionizationFT-photocurrent spectrum.

In another embodiment, the magnitude of electroluminescence emission ofa double heterostructure, single quantum well, or multiple quantum wellAlInGaN light emitting diode (LED) is measured as a function of infraredwavelength, wherein the preferred wavelength is the wavelength of peakemission intensity, and the resulting infrared spectrum is theinfrared-electroluminescence spectrum. The FTIR spectra are referred toas FT-photocurrent or FT-electroluminescence.

In use, the semiconductor device's electroluminescence or photocurrentis analyzed to determine a wavelength that increases carrierconcentration. For example, III-nitride materials represent a class ofwide bandgap semiconductors, with the most common gallium nitride GaNhaving an ambient temperature bandgap of approximately 3.4 electronvolts (eV), wherein the bandgap is defined as the distance between avalence band and a conduction band. As the temperature of an undopedsemiconductor device increases, the thermal energy of the electronsincrease, allowing more electrons to cross the bandgap from thepractically full valence band into the practically empty conductionband. In wide bandgap materials, such as GaN, the quantity of electronscrossing the bandgap is relatively small at room temperature. When anelectron is excited from the valence band to the conduction band itleaves behind a vacancy which behaves as a second carrier of positivecharge, referred to herein as a hole. As electrons drift through thesemiconductor in an electric field, holes drift in the oppositedirection. Holes and electrons created in this way are known asintrinsic charge carriers. In a doped semiconductor, the electrons orholes are excited not from the valence and conduction bandsrespectively, but rather from donor states, such as silicon, or acceptorstates, such as magnesium, in the forbidden gap, then the chargecarriers are extrinsic charge carriers, and the semiconductor isreferred to as n-type or p-type depending on whether the majority ofcarriers are electrons or holes respectively. A carrier concentration isthe number of charge carriers per unit volume. For wide bandgapsemiconductors, the band-to-band transition has the energy of visible orultraviolet light, while the donor and acceptor ionization energies havethe energy of infrared light. Whether intrinsic or doped, the product ofthe number of electrons and the number of holes in a particularsemiconductor is equal to a constant, i.e., the square of the intrinsicelectron (and hole) concentration.

Identifying 16 at least one wavelength corresponding to an acceptorionization in the device based on at least one of the photocurrent in alayer of p-type material and the electroluminescence emission from aheterostructure light emitting diode (LED) includes analyzing thephotocurrent or the electroluminescence to identify a wavelengthcorresponding to an optimal acceptor ionization wavelength, wherein theacceptor is a p-type dopant or an n-type dopant. In one embodiment, theexemplary wavelength has an energy between approximately 50 meV andapproximately 1000 meV. In another embodiment, the exemplary wavelengthhas an energy which is approximately 20% of a bandgap width for thesemiconductor device, such as 0.68 eV for a GaN semiconductor device.Identifying 16 at least one wavelength corresponding to an acceptorionization in the device based on at least one of the photocurrentemission and the electroluminescence emission also includes using aFourier transform hardware mathematical algorithm to identify at leastone wavelength corresponding to the acceptor ionization in thesemiconductor device. Alternatively, any acceptable mathematicalalgorithm can be used to identify at least one wavelength correspondingto the acceptor ionization. In another embodiment, the FTIR can identifya plurality of wavelengths corresponding to a plurality of acceptorionization energies.

FIG. 2 is a method 20 for increasing carrier concentration in asemiconductor device. Method 20 includes providing 22 a group IIInitride semiconductor device, determining 24 a wavelength that increasescarrier concentration in the semiconductor device, and directing 26 atleast one infrared light source, at the determined wavelength, into asemiconductor device, donor or acceptor, excitation band. In oneembodiment, method 20 includes providing 22 a group III nitridesemiconductor device, and determining 24 a wavelength that increasescarrier concentration in accordance with method 10 described herein.

FIG. 3 is a pictorial view of an infrared light source 30 and asemiconductor device 32, such as a light emitting diode, in an exemplaryembodiment of the invention. In one embodiment, infrared light source 30is a laser. In another embodiment, infrared light source 30 is variablyselected depending on the wavelength identified corresponding to a donoror an acceptor ionization in semiconductor device 32. In one embodiment,semiconductor device 32 is a forward-biased heterostructure lightemitting diode such as, but not limited to, an Indium Aluminum GalliumNitride (InAlGaN) device. In another embodiment, semiconductor device 32is a laser diode, GaN based bipolar junction transistor (BJT), a pindiode, and an ultraviolet emitter.

In use, infrared light source 30 is positioned such that a radiationbeam is directed toward semiconductor device 32. In one embodiment,infrared light source 30 is located at any position such that infraredlight source 30 and semiconductor device 32 are optically coupled. Inanother embodiment, a plurality of infrared light sources 30 areoptically coupled to semiconductor device 32. In one embodiment,infrared light source 30 and semiconductor device 32 are non-unitarydevices optically coupled. In one embodiment, infrared light source 30is a laser diode, such as, but not limited to, an edge emitter laserdiode, or a vertical emitter laser diode. In one embodiment,semiconductor device 32 receives the infrared radiation from the top(shown in FIG. 4). In another embodiment, semiconductor device 32receives the infrared radiation from the side (not shown). Radiatingsemiconductor device 32 from the top facilitates implementation of themethod since a lateral dimension of the p-type material in aheterostructure III-nitride light emitting device is typically twoorders of magnitude larger than a thickness. Alternatively, radiatingsemiconductor device 32 from the side may facilitate a larger fractionof infrared light being absorbed by the p-type material, since theinfrared optical path length in the p-type layer is two orders ofmagnitude longer (re: Beer's Law). In an alternative embodiment,infrared light source 30 and semiconductor device 32 are unitary, i.e.,fabricated as a single semiconductor device including infrared lightsource 30 and semiconductor device 32, such as, but not limited to aheterostructure light emitting diode and a laser diode. Infrared lightsource 30 is then directed, at the determined wavelength, into asemiconductor device excitation band. In a p-type layer, a plurality ofelectrons undergo photoexcitation from the valence band to acceptorlevels, i.e. approximately 0.1 eV to approximately 0.5 eV, therebyfreeing a plurality of holes in the valence band. After beingtransported by diffusion and drift into the active layers of the device,the plurality of holes then recombine radiatively with electrons fromthe n-type layer, thereby generating higher intensityelectroluminescence emission at a nominal bias. In an exemplaryembodiment, semiconductor device 32 emits an ultravioletelectroluminescence. In an alternative embodiment, semiconductor device32 emits a visible light electroluminescence.

In other words, semiconductor device 32 has a current applied to it thatcauses semiconductor device 32 to emit ultraviolet radiation at a normallevel when not exposed to infrared radiation. However, since most p-typedopants are too deep to fully excite into the valence band at operatingtemperatures, exposing semiconductor device 32 to infrared radiationfrom infrared light source 30 provides the photonic energy to facilitateexciting holes, in the p-type material, from deep acceptor levels intothe valence band. This excitation facilitates radiative recombination inthe active layers with electrons from the n-type material in theconduction band, which causes semiconductor device 32 to emitultraviolet radiation at a level greater than the normal level.

In use, directing light source 30 at semiconductor device 32 increasesan internal quantum efficiency and an external quantum efficiency of theheterostructure, but may reduce a plug efficiency of a combined system,i.e., infrared light source 30 and semiconductor device 32.

While the invention has been described in terms of various specificembodiments, those skilled in the art will recognize that the inventioncan be practiced with modification within the spirit and scope of theclaims.

1. A method for determining a wavelength to improve carrierconcentration in a semiconductor device, said method comprising:providing a group III nitride semiconductor device; irradiating thesemiconductor device with a broad band light source to generate asemiconductor device response, wherein the semiconductor device responsecomprises at least one of a photocurrent in a p-type layer and anelectroluminescence emission from a light emitting device; andidentifying at least one wavelength corresponding to an acceptorionization in the semiconductor device based on at least one of thephotocurrent and the electroluminescence emission.
 2. A method inaccordance with claim 1 wherein the semiconductor device comprises asemiconductor light emitting device (LED).
 3. A method in accordancewith claim 1 wherein the semiconductor light emitting device comprisesan Indium Aluminum Gallium Nitrogen (InAlGaN) heterostructure LED.
 4. Amethod in accordance with claim 1 wherein the semiconductor devicecomprises a laser diode.
 5. A method in accordance with claim 1 whereinidentifying at least one wavelength corresponding to an acceptorionization in the device comprises identifying at least one wavelengthhaving an energy between approximately 50 milli-electron volts andapproximately 1000 milli-electron volts.
 6. A method in accordance withclaim 1 wherein identifying at least one wavelength corresponding to anacceptor ionization in the device comprises identifying at least onewavelength having an energy approximately twenty percent of a bandgapwidth of the semiconductor device.
 7. A method for determining awavelength to improve carrier concentration in a semiconductor device,said method comprising: providing a group III nitride semiconductordevice, wherein said semiconductor device comprises a laser diode and anIndium Aluminum Gallium Nitrogen (InAlGaN) light emitting diode (LED);irradiating the semiconductor device with a laser to generate asemiconductor device response, wherein the semiconductor device responsecomprises at least one of a photocurrent and an electroluminescenceemission; and analyzing at least one of the photocurrent and theelectroluminescence emission with a mathematical algorithm to identityat least one wavelength corresponding to an acceptor ionization in thedevice, wherein said wavelength has an energy between approximately 50milli-electron volts and approximately 1000 milli-electron volts.
 8. Amethod for increasing carrier concentration in a semiconductor device,said method comprising: providing a group III nitride semiconductordevice; determining a wavelength that increases carrier concentration inthe semiconductor device; and directing at least one infrared lightsource, at the determined wavelength, into a semiconductor deviceexcitation band.
 9. A method in accordance with claim 8 whereindetermining a wavelength to improve carrier concentration in thesemiconductor device comprises: irradiating the semiconductor devicewith a broad band light source to generate a semiconductor deviceresponse, wherein the semiconductor device response comprises at leastone of a photocurrent and an electroluminescence emission; and analyzingat least one of the photocurrent and the electroluminescence emissionwith a mathematical algorithm to identify at least one wavelengthcorresponding to an acceptor ionization in the device.
 10. A method inaccordance with claim 8 wherein directing at least one infrared lightsource comprises directing a laser, at the determined wavelength, into asemiconductor device acceptor excitation band.
 11. A method inaccordance with claim 8 wherein the semiconductor device comprise asemiconductor light emitting device (LED).
 12. A method in accordancewith claim 8 wherein the semiconductor light emitting device comprisesan Indium Aluminum Gallium Nitrogen (InAlGaN) LED.
 13. A method inaccordance with claim 8 wherein the semiconductor device comprises alaser diode.
 14. A method for increasing carrier concentration in asemiconductor device, said method comprising: providing a group IIInitride semiconductor device, wherein said semiconductor devicecomprises a laser diode and an Indium Aluminum Gallium Nitrogen(InAlGaN) heterostructure light emitting diode (LED); irradiating thesemiconductor device with a broad band light source to generate asemiconductor device response, wherein the semiconductor device responsecomprises at least one of a photocurrent and an electroluminescenceemission; analyzing at least one of the photocurrent emission and theelectroluminescence emission with a mathematical algorithm to identifyat least one wavelength corresponding to an acceptor ionization in thedevice; and directing a laser, at the determined wavelength, into asemiconductor device excitation band.